Field of the Invention
The present invention relates to a compound having a structure that functions as a photo acid generator, a polymer compound having a repeating unit that functions as a photo acid generator, a resist composition containing the polymer compound, and a patterning process using the resist composition.
Description of the Related Art
A finer pattern rule has been recently required for Large-Scale Integrated circuits (LSI) with higher integration and higher processing speed. Under such circumstances, a far ultraviolet lithography and a vacuum ultraviolet lithography are promising for the next generation fine patterning technologies. Especially, a photolithography using an ArF excimer laser beam as a light source (an ArF lithography) is an indispensable technology for an ultrafine patterning process with a size of 0.13 μm or less.
The ArF lithography started to be used partially from production of 130-nm node devices, and then it has been used as a main lithography technology from production of 90-nm node devices. As a lithography technology for the next 45-nm node devices, a 157-nm lithography using F2 laser was initially considered as a promising technology. However, a delay in development thereof due to several problems was indicated. Then, an ArF immersion lithography rapidly emerged and is now in the stage of practical use. In the ArF immersion lithography, a liquid whose refractive index is higher than air, such as water, ethylene glycol, and glycerin, is placed between a projection lens and a wafer. This enables the number of aperture (NA) of the projection lens to be 1.0 or more, thereby attaining high resolution (for example, see Non-Patent Document 1). For this immersion lithography, a resist composition not readily eluting into water is required.
The ArF lithography requires a highly sensitive resist composition that can express a sufficient resolution with a small exposure dose to prevent degradation of a precise and expensive optical material. To obtain the highly sensitive resist composition, components having high transparency at a wavelength of 193 nm are usually selected as the components contained in the resist composition. For example, polyacrylic acid, derivatives thereof, norbornene-maleic anhydride alternating copolymer, polynorbornene, ring-opening metathesis polymers, and hydrogenated ring-opening metathesis polymers have been proposed as the base resin. These compounds bring a certain level of results in enhancing transparency of the resin alone.
In addition, a negative tone development with an organic solvent has recently attracted attention, as well as a positive tone development with an alkaline developer. The negative tone development can form an ultrafine hole pattern, which is not achievable by the positive tone development, by organic solvent development that uses a positive resist composition having high resolution. Moreover, an attempt to double the resolution by combining two developments of the alkaline development and the organic solvent development is under study.
As an ArF resist composition for the negative tone development with an organic solvent, conventional positive ArF resist compositions can be used. Patent Documents 1 to 3 disclose patterning processes using the conventional positive ArF resist compositions.
The resist compositions have developed with the rapid trend towards miniaturization in recent years, as well as the process technology, and various photo-sensitive acid generators have been studied. As the photo acid generator, sulfonium salts of triphenylsulfonium cation and perfluoroalkane sulfonate anion are commonly used. Unfortunately, these salts generate perfluoroalkanesulfonic acid, especially perfluorooctane sulfonic acid (PFOS), which has low decomposability, biological concentration, and toxicity, and thus are difficult to be used for the resist composition. Then, a photo acid generator that generates perfluorobutane sulfonic acid is currently used. However, when the photo acid generator that generates perfluorobutane sulfonic acid is used for the resist composition, a generated acid considerably diffuses, and thus high resolution is difficult to be achieved. To solve this problem, various alkanesulfonic acids partially substituted with fluorine and salts thereof are studied. For example, Patent Document 1 mentions, as the prior art, a photo acid generator that generates α,α-difluoroalkanesulfonic acid by exposure, more specifically a photo acid generator that generates di(4-tert-butylphenyl)iodonium-1,1-difluoro-2-(1-naphthyl)ethanesulfonate or an α,α,β,β-tetrafluoroalkanesulfonic acid. Although these generators are substituted with less fluorine, they contain no decomposable substituent such as an ester structure and thus are insufficient in view of environmental safety due to easily decomposability. Furthermore, there are other problems of limitation in the molecular design to change the size of the alkanesulfonic acid and cost of fluorine-containing starting materials.
Patent Document 4 discloses a photo acid generator having 2-acyloxy-1,1,3,3,3-pentafluoropropane-1-sulfonic acid. This generator has easily decomposability and advantageously environmental safety, and allows various design by changing an acyl group. In particular, introduction of a bulky substituent or a poplar group into the photo acid generator is effective in controlling acid diffusion. For example, a photo acid generator having 2-(1-adamantyloxy)-1,1,3,3,3-pentafluoropropane-1-sulfonic acid can reduce the acid diffusion to some extent. However, the resist composition disclosed in Patent Document 4 becomes unsatisfactory as the acid diffusion increases and affects lithography properties with a recent decrease in circuit line width. In particular, line width roughness (LWR) and critical dimension uniformity (CDU) significantly deteriorate by the effect of acid diffusion.
In this context, many attempts are made to incorporate a repeating unit that functions as a photo acid generator into the base resin so as to control the acid diffusion. For example, Patent Document 5 discloses a polymer compound obtained by polymerizing an acryloyloxyphenyldiphenyl sulfonium salt, and Patent Document 6 discloses polymerizing and thereby incorporating the acryloyloxyphenyldiphenyl sulfonium salt into the base resin to improve LWR of a polyhydroxystyrene resin. In these resins, however, the cation side bonds to the polymer compound, so that a sulfonic acid generated therefrom by irradiation with a high energy beam is similar to a sulfonic acid generated from the conventional photo acid generator. Thus, such resins are insufficient for controlling the acid diffusion. On the other hand, Patent Documents 7 and 8 disclose a polymer obtained by polymerizing a sulfonium salt and a resist composition containing a polymer whose skeleton is fluorinated, which improve LWR to some extent. However, these materials are still insufficient in lithography properties such as LWR, considering the trend towards miniaturization and finer patterning for the next 32-nm node and later one.